I used to add caster sugar to sweeten the juice (and some honey later on), but I have recently switched to using rock sugar 冰糖 (which the Chinese believe to have medicinal properties), and the resulting roselle juice tastes just like Ribena. It is slightly sweeter than my other version (using caster sugar), however I have to say this tastes a lot nicer. It is important not to boil it for too long, or else the juice will taste quite concentrated and sour.